1. Field of the Invention
This invention relates to substantially colorless, thermally colorable, indigo compounds. The compounds find use as color reactants in thermal sensitive materials.
The invention particularly relates to certain colorable dehydroindigo di(alkyl acylate) compounds and to their use in thermal sensitive marking materials.
This invention relates to thermal sensitive materials in film and sheet form, transparent and non-transparent. The invention more particularly relates to transparent marking materials incorporating colorable dehydroindigo di(alkyl acylate) compounds as a thermal sensitive color reactant. The invention more particularly relates to substantially opaque marking materials coated by dehydroindigo di(alkyl acylate) compounds.
The invention even more particularly relates to certain dehydroindigo di(alkyl acylate) compounds used in thermal coloration reactions in combination with certain color intensifying additives.
2. Description of the Prior Art
U.S. Pat. No. 3,666,525 discloses a heat sensitive copying sheet which utilizes a spiro compound and an organic sulfonic acid in a thermofusible material. A dispersion of the spiro compound, acid, and thermofusible material is coated onto a support material and marks are generated by application of sufficient heat to melt the thermofusible material and place the spiro compound and the acid in reactive contact.
U.S. Pat. No. 3,594,208 discloses a heat sensitive transparency sheet for light projection wherein two color reactant components are individually coated onto a substrate carrier using different polymeric binder materials. Solvents are selected to eliminate reactive contact of the components during coating operations.
U.S. Pat. No. 3,914,510 issued Oct. 21, 1975 discloses a heat sensitive transparency sheet having individual color reactant layers including a base layer of binder and acid reactant and a surface coating of binder and chromogen. The binder of the base layer is hardened by crosslinking to improve qualities of the transparency.